1. Overview
Communication is the transfer of information from one place to another.
This should be done
- as efficiently as possible
- with as much fidelity/reliability as possible
- as securely as possible  
Communication System: Components/subsystems act together to accomplish information transfer/exchange.

2. Elements of a Communication System
Output message
Input message
Input Transducer
Transmitter
Channel
Receiver
Output Transducer

3. Input Transducer: The message produced by a source must be converted by a transducer to a form suitable for the particular type of communication system.
Example: In electrical communications, speech waves are converted by a microphone to voltage variation.
Transmitter: The transmitter processes the input signal to produce a signal suits to the characteristics of the transmission channel.
Signal processing for transmission almost always involves modulation and may also include coding. In addition to modulation, other functions performed by the transmitter are amplification, filtering and coupling the modulated signal to the channel.

4. Channel: The channel can have different forms: The atmosphere (or free space), coaxial cable, fiber optic, waveguide, etc.
The signal undergoes some amount of degradation from noise, interference and distortion
Receiver: The receiver’s function is to extract the desired signal from the received signal at the channel output and to convert it to a form suitable for the output transducer.
Other functions performed by the receiver: amplification (the received signal may be extremely weak), demodulation and filtering.
Output Transducer: Converts the electric signal at its input into the form desired by the system user.
Example: Loudspeaker, personal computer (PC), tape recorders.

5. To be transmitted, Information (Data) must be transformed to electromagnetic signals.

6. Electromagnetic Waves.

7. Electromagnetic Waves.

8. Electromagnetic Spectrum

9. Electromagnetic Spectrum

11. Communication systems
Digital
Analog
The block diagram on the top shows the blocks common to all communication systems

12. Remember the components of a communications system:
Input transducer: The device that converts a physical signal from source to an electrical, mechanical or electromagnetic signal more suitable for communicating
Transmitter: The device that sends the transduced signal
Transmission channel: The physical medium on which the signal is carried
Receiver: The device that recovers the transmitted signal from the channel
Output transducer: The device that converts the received signal back into a useful quantity

13. Analog Modulation
The purpose of a communication system is to transmit information signals (baseband signals) through a communication channel
The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer
For example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of Hz
The “hello” wave is a baseband signal:

14. Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation
Remember the radio spectrum:
For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band)
The FM radio system must operate with frequencies in the range of MHz (VHF band)
AM radio
FM radio/TV

15. Basic analog communications system
Baseband signal (electrical signal)
EM waves (modulated signal)
Transmitter
Input transducer
Transmission Channel
Modulator
EM waves (modulated signal)
Carrier
Baseband signal (electrical signal)
Receiver
Output transducer
Demodulator

16. Types of Analog Modulation
Amplitude Modulation (AM)
Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant
Frequency Modulation (FM)
Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant
Phase Modulation (PM)
Another form of analog modulation technique which we will not discuss

17. Amplitude Modulation Carrier wave Baseband signal Modulated wave
Amplitude varying-frequency constant

18. Frequency Modulation Carrier wave Baseband signal Modulated wave
Large amplitude: high frequency
Baseband signal
Small amplitude: low frequency
Modulated wave
Frequency varying-amplitude constant

19. Example Amplitude modulation

20. Figure AM band allocation

21. Figure Frequency modulation

22. Figure FM band allocation

23. Figure Phase modulation

24. AM vs. FM AM requires a simple circuit, and is very easy to generate.
It is simple to tune, and is used in almost all short wave broadcasting.
The area of coverage of AM is greater than FM (longer wavelengths (lower frequencies) are utilized-remember property of HF waves?)
However, it is quite inefficient, and is susceptible to static and other forms of electrical noise.
The main advantage of FM is its audio quality and immunity to noise. Most forms of static and electrical noise are naturally AM, and an FM receiver will not respond to AM signals.
The audio quality of a FM signal increases as the frequency deviation increases (deviation from the center frequency), which is why FM broadcast stations use such large deviation.
The main disadvantage of FM is the larger bandwidth it requires